An OFDM (Orthogonal Frequency Division Multiplexing) mechanism is used in a data communication system such as a wireless LAN system. In a wireless LAN system, various aspects such as communication distance through a transmission line or a shielding material therein determine whether it is possible to perform high-speed transmission or only low-speed transmission. In a transmission line of a wireless LAN system, an adaptive modulation technology is implemented in which a modulation technique is adaptively varied depending on the condition of the transmission line to enable high-speed transmission as much as possible without degrading communication quality parameters such as bit error rate (BER) and the like.
In the data communication system, the condition of the transmission line is obtained, e.g., by measuring an SIR (Signal to Interference power Ratio) in a known signal (preamble). A preamble can be designed from, e.g., a plurality of symbols by repeatedly arranging identical data as disclosed in Nonpatent Literature 1.
Nonpatent Literature 1: “Synchronization scheme of OFDM systems for high speed wireless LAN”, Journal of Institute of Electronics, Information and Communication Engineers (IEICE), Technical report RCS 97-210, 1998-01.